Digital signal processing has been proven to be very efficient in handling and manipulating large quantities of data. There are many products that are in common use such as wireless devices, digital cameras, motor controllers, automobiles, and toys, to name a few, that rely on digital signal processing to operate. Many of these products continuously receive information that is monitored and used to produce adjustments to the system thereby maintaining optimum performance. The data is often an analog signal that must be converted to a representative digital signal. For example, light intensity, temperature, revolutions per minute, air pressure, and power are but a few parameters that are often measured. Typically, an analog to digital (A/D) converter is the component used to convert an analog signal to a digital signal. In general, the conversion process comprises periodically sampling the analog signal and converting each sampled signal to a corresponding digital signal.
Many applications require the analog to digital converter(s) to sample at high data rates, operate at low power, and provide high resolution. These requirements are often contradictory to one another. Furthermore, cost is an important factor that directly correlates to the amount of semiconductor area needed to implement a design. One type of analog to digital converter that has been used extensively is a redundant signed digit (RSD) analog to digital converter. The RSD analog to digital converter typically comprises one or more RSD stages and a sample/hold circuit. In one embodiment, a sampled voltage is compared against a high reference voltage and a low reference voltage. The result of the comparison is used to determine a bit (1 or 0) from the RSD stage. A residue voltage is then generated that relates to the sampled voltage less the voltage value of the extracted bit. The residue voltage is then provided to another RSD stage or fed back in a loop to continue the conversion process to extract bits until the least significant bit is generated.
In most applications, the analog signal that is to be converted is a single ended signal and can have values ranging from ground to the supply voltage. The A/D converter RSD circuitry in turn uses differential signaling to provide noise isolation, increase dynamic range, reduce errors due to charge injection and improve power supply noise rejection. The differential signaling must be in the operational range of the A/D converter sub functions which is less than the supply voltage and more than ground potential. The purpose of the sample and hold function is to convert the full range analog input signal into a scaled differential signal during the sampling process. For the A/D converter RSD circuitry to perform a conversion on the scaled input signal, it must use a voltage reference that has been scaled appropriately. Circuits that are used to scale a reference voltage typically use a resistor ladder. The reference voltage is buffered and then filtered using bypass capacitors for noise considerations. Such circuits introduce a number of problems that result in a mis-match in scaling between the reference generator and the interface function. This mis-match, between the two scaling functions results in either an A/D converter transfer gain error, transfer offset error or both. Additionally, the scaled output voltages require large capacitors that are typically required to be implemented external to an integrated circuit chip due to their required size. Therefore, additional integrated circuit pin count must be dedicated to implement the scaling function. As a result, the generation of scaled reference voltages for use in an RSD data converter introduces error, requires additional circuitry and package support pins.